Nine (9) clinic visits are required to complete this clinical trial. During visits to the clinic, each child will be evaluated by members of the research team to determine the child?s strength. Strength will be measured in different ways: manual testing, quantitative testing, and the usual measures used during visits to the neurologist (child will be asked to rise from the floor, walk across room, etc.). Manual testing is the way strength is usually measured by your child?s doctor. The quantitative muscle strength test (QMT) is a mechanical way to measure strength: the child sits/lays on an examining table and is asked to pull/push a strap. This test will determine the strength of different muscle groups in kilograms of force. An electrocardiogram (ECG, a heart test) will be performed during screening visit 2 and month 6 and blood (a small amount equal to about 2 to 3 tablespoons) will be drawn on screening visit 2 and months 1, 3 and 6. Urine tests will be done at screening visit 2 nd months 1, 3 and 6 to monitor kidney function.